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United States Patent |
6,174,421
|
Schumann
|
January 16, 2001
|
Sensor for determining the concentration of oxidizable elements in a gas
compound
Abstract
Sensor for measuring the concentration of oxidizable constituents in a gas
mixture, in particular for measuring one or more NO.sub.x, CO, H.sub.2
gases, and preferably unsaturated hydrocarbons, by measuring the voltage
between a measuring electrode a reference electrode or by measuring the
voltage between two measuring electrodes. A porous solid electrolyte makes
it possible to dispense with a reference gas atmosphere, thus providing
greater miniaturization and simplifying the design. The selectivity toward
individual measuring gas constituents can be improved by selecting the
measuring electrode materials, in particular by using semiconductors.
Inventors:
|
Schumann; Bernd (Rutesheim, DE)
|
Assignee:
|
Robert Bosch GmbH (Stuttgart, DE)
|
Appl. No.:
|
202313 |
Filed:
|
March 22, 1999 |
PCT Filed:
|
March 3, 1997
|
PCT NO:
|
PCT/DE97/00380
|
371 Date:
|
March 22, 1999
|
102(e) Date:
|
March 22, 1999
|
PCT PUB.NO.:
|
WO97/47962 |
PCT PUB. Date:
|
December 18, 1997 |
Foreign Application Priority Data
| Jun 11, 1996[DE] | 196 23 212 |
Current U.S. Class: |
204/424; 204/291; 204/426 |
Intern'l Class: |
G01N 027/407 |
Field of Search: |
204/424,426,427,428,429,425,291
205/783.5,784,784.5,785,781
429/33
|
References Cited
U.S. Patent Documents
5520789 | May., 1996 | Takahashi et al. | 204/424.
|
5630920 | May., 1997 | Friese et al. | 204/424.
|
5814719 | Sep., 1998 | Suzuki et al. | 73/23.
|
6022464 | Feb., 2000 | Schumann | 204/424.
|
Primary Examiner: Warden, Sr.; Robert J.
Assistant Examiner: Olsen; Kaj K.
Attorney, Agent or Firm: Kenyon & Kenyon
Claims
What is claimed is:
1. A sensor for measuring a concentration of at least one oxidizable
constituent in a gas mixture, comprising:
a reference electrode catalyzing an establishment of a thermal equilibrium
of the gas mixture;
an ion-conducting solid electrolyte having at least one pore, the solid
electrolyte including additives which catalyze oxidation of one of:
the at least one oxidizable constituent, and
the at least one oxidizable constituent that interferes with a reference
gas in the gas mixture;
at least one measuring electrode exposed to the at least one oxidizable
constituent, the at least one measuring electrode being substantially
unable to catalyze the establishment of the thermal equilibrium of the gas
mixture; and
a flat electrically insulating substrate,
wherein the reference electrode, the solid electrolyte and the at least one
measuring electrode are situated in vertical layers on a large surface of
the substrate.
2. The sensor according to claim 1, wherein:
the at least one oxidizable constituent includes at least one of NO.sub.x,
CO, H.sub.2 gas and unsaturated hydrocarbons.
3. The sensor according to claim 1, wherein a size of the at least one pore
is between 10 nm and 100 .mu.m.
4. The sensor according to claim 1, wherein:
the at least one measuring electrode has at least one pore.
5. The sensor according to claim 4, further comprising:
at least two measuring electrodes.
6. The sensor according to claim 4, wherein:
the reference electrode is composed of a first material;
the at least one measuring electrode is composed of a second material; and
the solid electrolyte includes at least two vertically arranged layers, a
first layer of the at least two layers facing the reference electrode and
composed of a third material which includes the first material, a second
layer of the at least two layers facing the at least one measuring
electrode and composed of a fourth material which includes the second
material, the first layer being different from the second layer.
7. The sensor according to claim 1, wherein the at least one measuring
electrode includes at least one semiconductor.
8. The sensor according to claim 7, wherein the at least one semiconductor
is doped with at least one of an acceptor and a donor.
9. The sensor according to claim 8, wherein a first concentration of the
donor is higher than a second concentration of the acceptor.
10. The sensor according to claim 8, wherein the donor is composed of a
first element having a first valence, the at least one semiconductor being
composed of a second element having a second valence, the first valence
being higher than the second valence.
11. The sensor according to claim 10, wherein the donor is composed of at
least one of tantalum and niobium.
12. The sensor according to claim 8, wherein:
the at least one semiconductor is doped with the acceptor, the acceptor
including at least one transition element.
13. The sensor according to claim 12, wherein the at least one transition
element is at least one of nickel, copper, cobalt and chromium.
14. The sensor according to claim 13, wherein the at least one transition
element is at least one of nickel, copper, cobalt and rare earths.
15. The sensor according to claim 12, wherein:
the acceptor is incorporated into the at least one semiconductor as one of
a solid solution and a segregated constituent.
16. The sensor according to claim 8, wherein a concentration of at least
one of the acceptor and the donor is between 0.01% and 25%.
17. The sensor according claim 16, wherein the at least one semiconductor
is composed of 0.5% to 15% niobium and 0.25% to 7% nickel.
18. The sensor according claim 16, wherein the at least one semiconductor
is composed of 7% niobium and 3% nickel.
19. The sensor according claim 7, wherein the at least one semiconductor is
composed of one of an oxide, a single-phase mixed oxide and multi-phase
mixed oxide.
20. The sensor according claim 19, wherein the at least one semiconductor
is composed of one of a rutile oxide, a dirutile oxide and a further
oxide, the further oxide including a mixture of the rutile oxide and the
dirutile oxide.
21. The sensor according to claim 19, wherein the at least one
semiconductor is composed of a titanium oxide.
Description
FIELD OF THE INVENTION
The present invention relates to a sensor for measuring the concentration
of oxidizable constituents in a gas mixture, in particular for measuring
one or more of the gases NO.sub.x, CO, H.sub.2, and preferably unsaturated
hydrocarbons.
BACKGROUND INFORMATION
Elevated concentrations of oxidizable constituents, in particular, of
NO.sub.x, CO, H.sub.2, and hydrocarbons, can occur in the exhaust gases of
spark-ignition and diesel engines, internal-combustion machines and
incineration plants, e.g. as the result of a component malfunction such as
an injection valve or as the result of incomplete combustion. To optimize
the combustion reaction, it is therefore necessary to determine the
concentration of these exhaust gas constituents. A method for measuring
oxidizable gases is described in Unexamined Japanese Patent Application
No. 60-61654, in which a stoichiometric reaction with oxygen takes place
at a first measuring electrode made of platinum-class metals, and
quasi-equilibrium states are established at one or more additional
metallic measuring electrodes with reduced catalytic activity for the
oxygen equilibrium reaction. Nernst voltages E1 and E2 are measured
between the measuring electrodes and a reference electrode, which is
exposed to a reference gas having a constant oxygen partial pressure, and
the concentrations of the gas constituents calculated from the difference
between these voltages on the basis of calibration curves.
SUMMARY OF THE INVENTION
A sensor according to the present invention provides greater
miniaturization, a simplified design, and more cost-effective production
than a conventional sensor. This is achieved according to the present
invention by the fact that the solid electrolyte is porously sintered. The
molecules of the measuring gas can thus diffuse through the
solid-electrolyte pores and reach the reference electrode, where
thermodynamic equilibrium is established. This exposes the reference
electrode to an oxygen partial pressure which corresponds to the
thermodynamic equilibrium. It is therefore not necessary to supply a
reference gas, which greatly simplifies the sensor layout.
The thermodynamic equilibrium can also be advantageously established even
in the solid electrolyte by selecting a catalytically active solid
electrolyte material. A particular advantage of this is that it enables
the gases interfering with the reference signal to be selectively
oxidized, simplifying signal analysis or even making it possible in the
first place. Not only the solid electrolytes but also the measuring
electrodes can be suitably porous, which further improves the diffusion of
the measuring gas molecules to the reference electrode.
Mixing additives made of the same substances as the electrodes into at
least one layer of the solid electrolyte which lie adjacent to the
electrodes improves electrode adhesion and thus also the service life of
the sensor. It is also especially advantageous to construct the measuring
electrodes from semiconductors, which can significantly increase
selectivity toward the individual gas constituents to be detected.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 shows a cross-section of a sensor according to the present
invention.
DETAILED DESCRIPTION
FIG. 1 shows a cross-section of a sensor according to the present
invention. One large surface of an insulating flat ceramic substrate 6
contains, in vertically arranged layers, a reference electrode 5,
preferably made of platinum, a porous solid electrolyte 4, measuring
electrodes 1 and 2, and a gas-permeable protective layer 3. A heating
device 7 with a cover 8 is mounted in the opposite large surface.
To measure the concentration of oxidizable constituents in exhaust gases,
the sensor is heated to a temperature between 300 and 1,000.degree. C.,
advantageously to 600.degree. C., using heating device 7.
The measuring gas diffuses through the porous solid electrolyte to
reference electrode 5, which catalyzes the establishment of the oxygen
equilibrium potential. Measuring electrodes 1, 2 are designed to have
reduced catalytic activity in the oxygen equilibrium reaction. On at least
one measuring electrode, the sensor generates a cell voltage via the
oxygen ion conducting solid electrolyte by a first half-cell reaction
initiated with the help of the reference electrode and a second half-cell
reaction influenced by the oxidizable gas constituents to be measured. The
concentrations of the gas constituents is determined from the voltage
values on the basis of the calibration curves.
In its simplest form, the sensor according to the present invention can be
used with one reference electrode which catalyzes the establishment of gas
mixture equilibrium and with one measuring electrode which cannot or can
only slightly catalyze the establishment of gas mixture equilibrium.
However, it is also possible to install two measuring electrodes, as shown
in FIG. 1, or even multiple electrodes, each with different catalytic
activity for establishing oxygen equilibrium states. The measuring
electrodes then respond with different voltages in relation to the
reference electrode, depending on the type of gas.
In arrangements with two or more measuring electrodes with different
catalytic activities, it is also possible to evaluate voltages between the
measuring electrodes in order to measure oxidizable gases. Measuring
voltages between electrodes that are on the same plane and positioned
equidistant from the heating device, such as electrodes 1 and 2 in FIG. 1,
also disables the Seebeck effect.
In arrangements with at least two measuring electrodes, it is also possible
to completely or at least partially compensate for cross-sensitivity of a
first measuring electrode us using the signal of an additional measuring
electrode by adjusting the sensitivity of this additional measuring
electrode to the interfering gas constituents.
According to an additional embodiment, the solid electrolyte is designed in
such a way, e.g. by mixing in 0.01% to 10% by volume of platinum powder
into at least one layer of the solid electrolyte facing the reference
electrode, that the solid electrolyte catalytically converts the gases to
be measured so that only the gases corresponding to the thermodynamic
equilibrium reach the reference electrode, or that the solid electrolyte
converts only the gases interfering with the reference signal.
According to an additional embodiment, one or more measuring electrodes, in
addition to the solid electrolyte, are porous, thereby facilitating the
diffusion of gas to the reference electrode.
Concerning the measuring electrode composition, it is possible to select
metal electrode substances like those described, for example, in
Unexamined Japanese Patent Application No. 60-61654, or semiconductors
with a high specific sensitivity toward certain oxidizable gases.
Especially suitable are semiconducting oxides or mixed oxides, which can
be doped with an acceptor and/or donor having a concentration, for
example, of between 0.01% and 25%. The acceptor is incorporated into the
semiconductor as, for example, a solid solution or a segregated
constituent. The high sensitivity, e.g. of acceptor- and donor-doped
n-type titanium oxide, in particular to unsaturated hydrocarbons, is due
to the adsorbent interaction between the orbitals of the pi bonds of the
unsaturated hydrocarbons and the acceptor sites on the semiconductor
surface.
To prevent the conductivity-reducing acceptor component from becoming fully
electronically active, it is advantageous to add to the electrode a
conductivity-increasing donor, in particular in a higher concentration
than the acceptor.
The following example describes a method for producing a sensor according
to the present invention: rutile doped with 0.5% to 15%, for example 7%,
niobium and 0.25% to 7%, for example 3%, of one of transition metals
nickel, copper or iron is screen-printed in a 30 .mu.m layer onto a
substrate on which are mounted a reference electrode, preferably made of
platinum, and, on top if this, a solid electrolyte layer. A heating device
is attached to the opposite side of the substrate. The sensor is sintered
for 90 minutes at 1,200.degree. C. with a heating/cooling ramp of
300.degree. C./hour. After sintering, the solid electrolyte has pores
ranging in size from 10 nm to 100 .mu.m.
An attached platinum conductor path, which is insulated from the solid
electrolyte and contacts only the measuring electrode, is used to measure
the voltage between the reference and rutile electrodes at the cell formed
in this manner with a resistance of 1 MOhm. In doing this, the sensor is
heated by its heating device to a temperature of 600.degree. C. A
simulated exhaust gas containing 10% oxygen, 5% water, and 5% carbon
dioxide, as well as 30 ppm sulfur dioxide, is used as the measuring gas.
Oxidizable gases are then added in the quantities shown in the table.
For comparison, the last line in the following table shows the voltage
values for a mixed-potential electrode made of 20% gold and 80% platinum,
representing a measuring electrode according to the related art.
TABLE
Voltage values (in mV) as a function of the concentration
of oxidizable gases and the composition of the measuring electrode
Rutile electrode
with 7% Nb and 3% Reference
oxidizable gases Voltages in mV electrode, 20%
(ppm) Ni Cu Fe Au and 80% Pt
Propene 460 150 45 60 320
180 120 36 47 280
90 90 27 35 180
H2 460 30 12 20 500
180 17 6 10 450
90 5 3 4 380
CO 460 40 3 16 70
180 15 -- 7 35
90 7 -- 6 23
The table shows that a rutile semiconductor electrode with 7% niobium as
the donor and 3% nickel as the acceptor demonstrates the greatest
sensitivity to propene as the conductive substance. The gold-platinum
system known from the related art, on the other hand, shows especially
high cross-sensitivity to hydrogen.
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